I have to say that my experience with the Panasonic 100-300 has been excellent and I use it with one of two Olympus E-M1 bodies. It focuses quick and very sharp with the lens IS switched off and IBIS on. That camera/lens combo has earned me many wildlife images accepted for exhibitions around the world. It is neither splash or dustproof but has served me well from South America to the wet areas of Botswana and the deserts of Namibia without any problems whatsoever.
I did have the mZuiko 75-300 lens which just did not perform in the same way for either focusing accuracy or image quality. My Panasonic DG Leica 45mm prime lens is also a trusted favourite as it makes superb images with a particularly smooth bokeh. It deals with specular highlights particularly well and this can sometimes be a problem when making images of wildlife in their natural environment in the bush. I am hoping that the Panaleica DG 100-400 will be similarly blessed!
The Premium and Pro lenses from Olympus are usually clinically sharp but tend to make a "colder"image than the Panaleica lenses in my eyes. To put it another way they are more 'artistic' in their rendering. Each has it's own uses as I would use the mZuiko 60mm in preference to the Panaleica 45 macro for fine detail work with insects as an example. They are all just tools for the job in hand and your hands may be quite different!
E-M5 and Panaleica DG 45mm lens also used for the following image:
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